My choices are NetBSD 6.0, OpenBSD 5.1 (5.2 wasn't released a few days ago) and OpenIndiana.

After a few failed attempts of trying raidframe on NetBSD I gave up, although I did actually have it setup according to the documentation it wouldn't boot properly.

Next I used softraid on OpenBSD and it was easy to setup and only took a minute to do so before installing. After 24 hours my rebuild of the RAID-1 was at 42%.

My third install was OpenIndiana and installed to one drive. Then attempted to mirror and had an error saying I can't use an EFI labeled disk. Quick search led me to a document and 5 minutes later I started resilvering. These were the results: